The present invention relates to a range finding device used in photographic cameras or video cameras.
Most recent photographic cameras, in particular compact cameras of the type having a lens shutter, are equipped with automatic focusing devices. Such an automatic focusing device generally comprises a photoelectric range finding device and a lens positioning mechanism. The photoelectric range finding device detects the reflected light from a subject in the field to provide a subject distance signal according to the distance of the subject. Responsive to the subject distance signal, the lens positioning mechanism moves the taking lens to a lens position in which the taking lens is suitably focused on the subject.
One such range finding device which is well known is an active type range finding device in which a line of light is projected onto a subject and reflected from the subject. A photoelectric light sensor receives the reflected light from the subject to provide a subject distance signal corresponding to the distance of the subject. The range finding device using such a line of light has no need to be exactly aimed at the subject during range finding. In this regard, the range finding device using a line of light is advantageous in comparison with the type using a spot of light which has to be exactly aimed at the subject to be photographed. Furthermore, the range finding device using a line of light can prevent the range finding device from merely detecting a space between two subjects standing side by side. For these reasons, it is desirable to provide such an active type range finding device using a line of light on simplified compact cameras.
Meanwhile, as long as the range finding device provides only one subject distance signal, the taking lens can be correctly adjusted to a corresponding lens position without confusion. However, because there are often various subjects such as a principal subject, secondary subjects and a background in the field of view, the range finding device may provide more than one subject distance signal. Consequently, it becomes hard to correctly adjust the taking lens to a position in which the taking lens is focused on the principal subject.
In an attempt at overcoming this problem, there has been proposed an improved automatic focusing device in which the taking lens is adjusted so as to give priority to the subject at the shortest distance when more than one subject distance signal is provided. This is the so-called short distance priority system.
There is also, as is disclosed in Japanese Patent Unexam. Publs. Nos. 59-146,028 and 59-146,029, another short distance priority system in which the taking lens is adjusted so as to give priority to the subject at the shortest distance of all the possible subjects, but only in consideration of the field depth thereof. According to this short distance priority system, when there are plural subjects in the field, the taking lens is focused on the subject at the shortest subject distance within a suitable subject distance range as defined by the field depth thereof and nearer to the shorter extremity of the range of field depth.
One problem with the short-distance-priority systems of range finding devices is that, when taking a picture of an intended principal subject that is disposed behind a short-distant secondary subject, the principal subject will be out of focus. In the system taught by the above-mentioned publications, when plural subject distance signals are provided, the shortest subject distance signal is used to shift the suitable subject distance range in which a subject is fairly brought into focus, toward a longer subject distance. Therefore, if the shortest subject distance signal is of the principal subject, what is fairly brought into focus is subjects or a scene other than the principal subject, and so the principal subject will be out of focus.
Another problem with the short distance priority system of range finding devices is that, because the larger the aperture of the taking lens becomes as the brightness of subject becomes less, the narrower will be the depth of field of the taking lens. The suitable subject distance range in which subjects are brought into sharp focus is narrowed when plural subject distance signals are provided. For these reasons, the probability that the principal subject will be out of focus becomes greater.